Mycoplasmas in pneumonias of sheep were investigated by survey and by pathogenicity experiments in specific pathogen free (SPF) and conventionally reared lambs. Glycolytic mycoplasmas isolated from sheep in Scotland were found, by comparison with the Queensland Y98 strain, using biochemical and serological tests, to be classifiable as Mycoplasma ovipneumoniae. Two serological tests indicated considerable intraspecific differences among strains. The ultrastructural morphology of M. ovivneumoniae was found to be typical of mycoplasmas. M. ovineumoniae and Mycoplasma arginini were the most commonly recovered mycoplasmas in a survey of housed sheep. Ureaplasmas, hitherto not reported to occur in the ovine respiratory tract, were also isolated. M. ovipneumoniae was recovered from the lungs of most lambs necropsied: its presence with P teurell & haemolvtica was consistently associated with proliferative exudative (P.E.) pneumonia, the incidence of which increased with age. M. arginini recoveries were not correlated with any specific hiatopathological changes. Endobronchiai inoculation with M. ovipneumoniae induced lung changes in only a proportion of SPF lambs, although the lungs of almost all animals became colonised. Lesions were similar to, but milder than, respiratory mycoplasmosea of other species of animals. In-contact transmission produced nasal but not pulmonary infection, and no lung lesions. Attempts to enhance the pathogenic effects of M. ovipneumoniae by sensitization through prior infection were unsuccessful. Endobronchial inoculation of SPF lambs with M. arginini produced neither lung colonisation nor lesions, although the upper respiratory tract became infected. The effects of endobronchial administration of lung homogenates, which were prepared from naturally-occurring P.E. pneumonia and which contained principally M. ovipneumoniae and P. haemolvtica were simulated by mixed cloned cultures of M. ovipneumoniae, M. arginini and P. haemolytica administered to conventionally reared sheep by the same route. Clinical signs and P.E. pneumonia were produced in almost all animals. M. ovipneumoniae was the only consistently recovered organism. P. haemolytica did not establish in the lungs of sheep unless combined with M. ovipneumoniae. This combination produced lesions of P.E. pneumonia. However, P.E. pneumonia was also observed in some animals inoculated with M. ovipneumoniae alone.